镁合金表面铈盐掺杂硅烷膜的腐蚀电化学行为

目的为进一步提升镁合金表面常规硅烷膜的耐蚀性能。方法在γ-氨丙基三乙氧基硅烷溶液中掺杂0.50 g/L硝酸铈,采用简单化学浸渍处理,在AZ91D镁合金基体表面制备了铈盐掺杂硅烷膜。借助扫描电子显微镜(SEM)观察了铈盐掺杂前后硅烷膜的表面微观形貌,通过开路电位-时间曲线、电化学交流阻抗谱(EIS)和中性盐雾试验(NSS)研究了铈盐掺杂对5%Na Cl溶液中硅烷膜耐蚀性能的影响。结果铈盐掺杂硅烷膜比普通硅烷膜更厚且平整,其致密性、均匀一致性较好,完全覆盖了镁合金基体,已看不到磨痕。铈盐掺杂硅烷膜的稳定电位约为-1.31 V,且需要的稳定时间最长。铈盐掺杂硅烷膜具有更大的低频阻抗数值,有效遏制了侵蚀性粒子向镁合金基体的迁移和扩散,避免了镁合金基体发生阳极溶解反应。结论采用向硅烷溶液中添加硝酸铈的方法,能够在AZ91D镁合金表面制备出铈盐掺杂硅烷膜。由于铈离子在某种程度上修复了硅烷膜层中的微裂纹和缺陷,显著提升了硅烷膜的耐蚀能力。     

镁合金阳极氧化膜在NaCl溶液中的腐蚀行为

利用盐水浸泡实验研究了AZ91D镁合金阳极氧化膜层在3．5％NaCl溶液中的腐蚀行为。结果表明：AZ91D镁合金阳极氧化膜层不论封闭与否,在中性NaCl溶液中浸泡出现第一个腐蚀点后,膜层表面均很少再出现新的腐蚀点,而是原有的腐蚀点向纵、横两个方向扩展形成腐蚀坑,表面呈“树枝”状腐蚀形貌;浸泡溶液的pH值对阳极氧化膜层的耐蚀性影响很大,酸性溶液中的腐蚀速率明显大于中、碱性溶液的;随浸泡溶液温度的升高阳极氧化膜层的腐蚀速率加快。据此,提出了AZ91D镁合金阳极氧化膜层在NaCl溶液中腐蚀过程的模型。     

Effect of HCl pre-treatment on corrosion resistance of cerium-based conversion coatings on magnesium and magnesium alloys

The corrosion protection afforded by a cerium conversion coating, formed by immersion in a solution containing rare earth salt and hydrogen peroxide, on pure magnesium and two magnesium alloys, AZ91 and AM50, has been studied. The effect of HCl pre-treatments on the morphology and on the corrosion resistance of the cerium conversion layer was investigated. A thicker and more homogeneous distribution of the conversion coating was obtained when the sample surface was pre-treated with acid. Higher amounts of cerium on the surface of the pre-treated samples were detected. The cerium conversion coating increased the corrosion resistance of the alloys because it ennobled the corrosion potential and decreased both the anodic and cathodic current. The acid pre-treatment further increased the corrosion resistance of the coated alloys. After five days of immersion in chloride environment the untreated samples showed localized corrosion while the chemical conversion coated samples appeared unaffected.     

Corrosion Performance of Anodized AZ91D Magnesium Alloy: Effect of the Anodizing Potential on the Film Structure and Corrosion Behavior

Die-cast AZ91D magnesium alloy samples have been submitted for anodizing at different potentials. Anodizing was conducted in an environmentally friendly solution which comprised 3 M KOH + 1 M Na₂SiO₃ at room temperature. The surface treatment was performed electrolytically at four different potentials: 3, 5, 8, and 10 V. The corrosion resistance was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization curves obtained after 7 days of immersion in a 3.5 wt.% NaCl solution at room temperature. The porosity of the anodic films was estimated by means of the linear polarization method. SEM images revealed that the surface oxide was thicker for the anodic films obtained at 3 and 5 V. The films obtained at these potentials were more porous than those formed at 8 and 10 V. The results showed that the thickness of the anodic film had a significant effect on the corrosion behavior of the AZ91D, whereas the influence of the initial porosity was not significant.     

Corrosion and self‐healing behaviour of AZ91D magnesium alloy in ethylene glycol/water solutions

Corrosion behaviour of magnesium alloy‐based engine parts in cooling system is an urgent fundamental issue in automotive field where magnesium alloys are increasingly used. In the present work, the corrosion behaviour of AZ91D magnesium alloys in various ethylene glycol/water solutions was studied by electrochemical measurements and immersion tests at room temperature. The surfaces of the samples after immersion tests were examined using scanning electron microscope (SEM) and X‐ray diffraction (XRD). The results showed that the corrosion rates of AZ91D magnesium alloys decreased with the increase of ethylene glycol concentration in ethylene glycol/water solutions and the corrosion process was dominated by pitting corrosion. A continuous protective film transferred from corrosion products was formed on the corroded surface after sufficient immersion duration in ethylene glycol/water solutions, which is able to heal the corrosion pits. The self‐healing behaviour inhibited the further corrosion of AZ91D magnesium alloy.     

镁合金表面电镀铝前处理工艺的研究

在镁合金表面电镀铝，必须对其表面进行前处理。采用氢氟酸对AZ61镁合金进行钝化处理，利用X射线衍射仪、涡流测厚仪、金相显微镜、扫描电镜和M273A电化学测试系统对钝化处理得到的转化膜的物相、厚度、表面形貌和耐蚀性进行测定。结果表明，氢氟酸处理得到的转化膜对镁合金基体有较好的保护作用，符合镀前处理的要求。     

Preparation and characterization of inorganic and organic coatings on AZ91D magnesium alloy with electroless plating pretreatment

In this paper, a protective coating scheme was applied for the corrosion protection of AZ91D magnesium alloy. Electroless Ni coating (EN coating) as bottom layer, electrodeposited Ni coating (ENN coating), and silane‐based coating (ENS coating) as top layer, respectively, were successfully prepared on AZ91D magnesium alloy by combination techniques. Scanning electron microscopy and X‐ray diffraction were employed to investigate the surface and phase structure of coatings, respectively. The electrochemical corrosion behaviors of coatings in neutral 3.5 wt% NaCl solution were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. The corrosion testing showed that the three kinds of coatings all could provide corrosion protection for AZ91D magnesium alloy to a certain extent, and the corrosion resistance of ENN and ENS was superior to EN. In order to further study the corrosion protection properties of ENN and ENS, a comparative investigation on the evolution of EIS of ENN and ENS was carried out by dint of immersion test in neutral 3.5 wt% NaCl solution. The results indicated that, compared with ENN, the ENS could provide longer corrosion protection for AZ91D magnesium alloy. It is significant to determine the barrier effect of each coating, which could provide reference for industry applications.     

Effect of zinc immersion pretreatment on the electro-deposition of Ni onto AZ91D magnesium alloy

The effect of zinc immersion pretreatment on the quality and properties of Ni deposited layer on the AZ91D Mg alloy has been studied. The evolution of surface morphology, composition and microstructure of the AZ91D Mg alloy in the process of pretreatment was characterized by scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX) and X-ray diffraction (XRD). The results showed that the activation step results in the deposition of some composite membranes consisted of fluoride and phosphate or pyrophosphate formed preferentially in the α phase region of the Mg alloy surface, while zinc immersion leads to the deposition of a network-like Zn layer formed preferentially in the β phase region of the Mg alloy surface. Cyclic voltammogram, dynamic potential scanning polarization technique and chronopotentionmetry measurements showed that the deposited Zn layer can markedly improve the anti-corrosion ability of the Mg alloy substrate in the Ni-plating solution and improve the nucleation and growth of Ni onto the surface of the Mg alloy substrate. With the proper Zn immersion pretreatment, a compact Ni layer can be electro-deposited on the magnesium alloy. The deposited Ni layer shows good adhesion to the Mg alloy substrate. The Mg alloy specimens after deposition of Ni have good anti-corrosion properties in a neutral 3.5 wt.% NaCl solution.     

镁阳极氧化膜微观结构和防护性能的比较

借助SEM(扫描电子显微镜)、盐雾试验等技术，对以不同阳极氧化工艺在镁合金AZ91D表面所得膜层的微观结构及其综合防护性能(包括耐蚀性、耐磨性及结合力)进行了比较研究.结果发现，不同工艺条件下形成的阳极氧化膜在微观结构及性能上存在显著差异.环保型阳极氧化新工艺所得膜层的综合防护性能优于经典工艺，对基体金属能提供更为有效的保护.      

Electroless Ni-P layer with a chromium-free pretreatment on AZ91D magnesium alloy

A phosphate-manganese conversion film was proposed as the pretreatment layer between Ni-P coating and AZ91D magnesium alloy substrate, to replace the traditional chromium oxide plus HF pretreatment. The subsequent Ni-P deposited on the layer was also characterized by its structure, morphology, microhardness and corrosion-resistance. The pretreatment layer on the substrate not only reduces the corrosion of magnesium during Ni-P plating process, but also reduces the potential difference between the matrix and the second phase. Thus, a Ni-P coating with fine and dense structure was obtained on the AZ91D magnesium alloy, which shows better corrosion resistance than the Ni-P with chromium oxide plus HF as pretreatment.     

添加剂对AZ91D镁合金化学镀镍磷耐蚀性的影响研究

采用常温碱性超声波清洗、酸洗和活化的前处理工艺在AZ91D镁合金表面化学镀镍,研究了辅助络合剂氨基乙酸和加速剂巯基乙酸对镀层耐蚀性的影响。结果表明:镀层表面由Ni-P固溶体组成,呈现尖锐的晶态峰,磷原子与镍原子形成置换固溶体,表面平整致密,无明显缺陷。氨基乙酸含量对镀层耐蚀性和腐蚀速率影响不大,随着巯基乙酸含量的增加,镀层腐蚀电位先升高后降低,腐蚀速率整体比镁合金基体慢,镀层总体呈现钝化趋势,在4 g/L时腐蚀电位最高。     

AZ91D镁合金的均一化前处理与化学镀Ni-P

采用新型环保的均一化前处理工艺在AZ91D镁合金表面制备了化学镀Ni-P镀层。研究了前处理过程中AZ91D基体微观形貌、镀层沉积过程、成分和相结构。研究结果表明：基体表面的β相在前处理过程中被选择性去除,表面组织得到均一化,从而获得均匀致密的浸Zn层。Ni-P颗粒均匀形核生长,并最终形成致密的镀层。镀层具有优良的耐腐蚀性能。     

AZ91D镁合金微弧氧化膜在不同浓度NaCl溶液中的腐蚀行为(英文)

采用微弧氧化技术(MAO)在镁合金 AZ91D 表面制备微弧氧化陶瓷膜。利用电化学技术和浸泡实验研究该镁合金试样在不同浓度(0.1%,0.5%,1.0%,3.5%和 5.0%,质量分数) NaCl 溶液中的腐蚀行为。结果表明,试样的腐蚀速率随着氯离子浓度的升高而增大。在较高浓度(1.0%,3.5%和 5.0%)的 NaCl 溶液中的主要腐蚀形式是点蚀,而在较低浓度(0.1%和 0.5%)中是全面腐蚀。腐蚀过程可以分为两个阶段:亚稳态蚀点的出现和蚀点的生长。根据腐蚀过程中阻抗谱的特点,对镁合金微弧氧化膜试样在不同浓度 NaCl 溶液中浸泡 120 h 提出了不同的等效电路来模拟其腐蚀行为。     

Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings

The corrosion performance of WE43-T6 and AZ91D magnesium alloys with and without treatment by plasma electrolytic oxidation (PEO) was investigated by electrochemical measurements in 3.5 wt.% NaCl solution. For untreated WE43-T6 alloy, formation of a uniform corrosion layer (Mg(OH)₂) was accompanied by initial pits around magnesium-rare earth intermetallic compounds. The AZ91D alloy disclosed increased corrosion susceptibility, with localized corrosion around the β-phase, though the β-phase network phase acted as a barrier for corrosion progression. PEO treatment in alkaline phosphate electrolyte improved the corrosion resistance of WE43-T6 alloy only at the initial stages of immersion in the test solution. However, PEO-treated AZ91D alloy revealed a relatively high corrosion resistance for much increased immersion times, contrary to the relative corrosion resistances of the untreated alloys. The improved performance of the PEO-treated AZ91D alloy appears to be related to the formation of a more compact coating.     

The effect of NaHCO3 treatment time on the corrosion resistance of commercial magnesium alloys AZ31 and AZ61 in 0.6 M NaCl solution

This paper studies the chemical composition and structure of the conversion coatings formed during immersion on saturated aqueous NaHCO₃ solution treatment on the surface of as-received commercial AZ31 and AZ61 magnesium alloys with a view to a better understanding of their protective properties. The great uniformity and the significant increase in the amount of aluminium oxides and hydroxides observed on the surface of the conversion coating of the AZ61 alloy after 10 or 60 min of treatment (about 30% higher Al atomic contents) seems to improve the corrosion resistance.     

The corrosion performance of anodised magnesium alloys

The corrosion performance of anodised magnesium and its alloys, such as commercial purity magnesium (CP-Mg) and high-purity magnesium (HP-Mg) ingots, magnesium alloy ingots of MEZ, ZE41, AM60 and AZ91D and diecast AM60 (AM60-DC) and AZ91D (AZ91D-DC) plates, was evaluated by salt spray and salt immersion testing. The corrosion resistance was in the sequential order: AZ91D ≈ AM60 ≈ MEZ ? AZ91D-DC ? AM60-DC > HP-Mg > ZE41 > CP-Mg. It was concluded the corrosion resistance of an anodised magnesium alloy was determined by the corrosion performance of the substrate alloy due to the porous coating formed on the substrate alloy acting as a simple corrosion barrier.     

Biodegradation and cytotoxic properties of pulse anodized Mg alloys

Magnesium has the potential to be used as an implant material owing to its non-toxicity. On the other hand, magnesium alloys corrode rapidly in subcutaneous gas bubbles. Consequently, the approach of using magnesium alloys as a biodegradable biomaterial is not well established. Therefore, the aim of this study was to provide corrosion protection by anodizing to surface for a biodegradable material. Micro-arc oxidation by pulsed DC was applied to AZ91D and AZ31B, and the cell bioactivity was defined. The anodic film was characterized by XRD and SEM. The specific mass loss variation from immersion test and potentiodynamic electrochemical test was performed for the quantification of corrosion resistance. Although the AZ91D had better corrosion resistance properties but the result of the in vitro tests showed low cell viability compared with the AZ31B. The results of the cell staining and agar overlay test revealed the AZ31B group had good biocompatibility and a low corrosion rate. In this study, the surfaces of AZ91D and AZ31B showed the formation of a uniform film by pulse power anodization improving corrosion resistance. Also, the cytotoxicity of the materials was examined by the aluminum content change of compound metal.     

Effect of magnesium hydride on the corrosion behavior of an AZ91 magnesium alloy in sodium chloride solution

The effect of magnesium hydride on the corrosion behavior of an as-cast AZ91 alloy in 3.5 wt.% NaCl solution was investigated using gas collection method and potentiostatic test. The Pourbaix diagram of Mg–H₂O system was built using thermodynamic calculation. It was possible that magnesium hydride could form in the whole pH range in theory. The experimental results showed that at cathodic region, magnesium hydride formed on surface, which was the controlling process for the corrosion behavior of AZ91 alloy; at anodic region and free corrosion potential, magnesium hydride model and partially protective film model, monovalent magnesium ion model and particle undermining model were responsible for the corrosion process of AZ91 alloy.     

Evaluation of corrosion resistance of magnesium alloys in radiator coolants

Abstract

Coolant corrosion is a major drawback for the use of magnesium alloys in engine and cooling system, but the coolant is not normally intended to prevent corrosion of magnesium alloys. This research assessed the corrosion performance of two magnesium alloys, AZ91D and AM50A, in two newly formulated radiator coolants using immersion test, potentiodynamic polarisation test, and corroded surface analysis. Two coolants were named as Irgacool Plus L and Irgacool Plus S. C7, C8-organic acids and polycarboxylic acid were the main inhibitor species in Irgacool Plus L while Irgacool Plus S was formulated with C7, C8-organic acids and sebacic acid inhibitors. Corrosion rates of magnesium alloys decreased twice in Irgacool Plus L compared with Irgacool Plus S. AZ91D alloy had better corrosion resistance than AM50A alloy in both radiator coolants. Both alloys suffered corrosion due to microgalvanic coupling between cathodic β-Mg₁₇Al₁₂ intermetallic and anodic α-Mg matrix, and the presence of Al₈Mn₅ and Al₁₁Mn₄ intermetallics in AM50A led to further microgalvanic corrosion. A continuous network of β-Mg₁₇Al₁₂ phase and higher Al content α-Mg matrix accounted for better corrosion resistance of AZ91D alloy.     

模拟海洋环境中Ni-Co合金镀层对AZ91D镁合金的腐蚀防护研究

目的提高AZ91D镁合金的腐蚀防护性能。方法采用化学镀前处理在AZ91D镁合金表面制备一种保护性的Ni-Co合金镀层。分别采用环境扫描电镜(ESEM)、X射线衍射(XRD)和能量散射谱(EDS)分析合金镀层的表面形貌、微结构特点和化学成分。采用动电位极化(PC)和电化学阻抗谱(EIS),分析测试在模拟海洋环境(中性3.5%Na Cl溶液)中Ni-Co合金镀层对AZ91D镁合金的腐蚀防护性能。结果镁合金表面化学镀Ni-P镀层均匀覆盖,晶粒生长较致密,表面呈菜花状形貌,Ni-P镀层中P质量分数约为5.6%。Ni-Co合金镀层表面均匀且呈金字塔状形貌,形成了面心固溶体(FCC),镀层中Co质量分数约为31%。Ni-P镀层和Ni-Co合金镀层的厚度分别约为11μm和19μm。在模拟海洋(中性3.5%Na Cl溶液)环境中,镁合金裸基体、化学镀前处理Ni-P镀层、Ni-Co合金镀层的腐蚀电位分别为-1485、-372、-284 m V,其腐蚀电流密度分别是3.4×10-5、1.8×10-6、2.9×10~(-7) A/cm2,所拟合的电荷转移电阻分别为4.72×103、1.70×104、2.06×106?/cm2。结论化学镀前处理Ni-P镀层可为镁合金提供较好的腐蚀防护,Ni-Co合金镀层能够为镁合金提供更显著的腐蚀防护。